(kī'ə-nīt') A bluish-green to colorless triclinic mineral. Kyanite occurs as long, thin, blade-shaped crystals in metamorphic rocks. It is unique among minerals in having two grades of hardness, one along its length, and one along its width. It is a polymorph of andalusite and sillimanite, but can form at lower temperatures than either of these. Chemical formula: Al2SiO5.